India's sodic-soil problem: 6.7 million hectares to reclaim
ICAR-CSSRI Karnal estimates India has 6.7 million hectares of sodic soil and another 2.3 million hectares of saline-sodic mix — concentrated in Uttar Pradesh (1.4 Mha), Gujarat (1.0 Mha), Haryana (0.5 Mha), Rajasthan, Punjab, and pockets of western Bihar. These are alluvial floodplain soils where high water-table conditions, sodium-rich groundwater, and inadequate drainage have accumulated Na⁺ on the cation-exchange complex. When the exchangeable sodium percentage (ESP) climbs above 15 the soil becomes "sodic": clay particles disperse, structure collapses, infiltration drops from cm/hr to mm/hr, and seedlings cannot establish. Yields fall to one-third or zero; the field is colloquially "usar" or "rehi" land — abandoned reh-encrusted plots visible from the Lucknow-Allahabad railway line.
How gypsum reclaims sodic soil
Gypsum (CaSO₄·2H₂O) supplies Ca²⁺ that displaces Na⁺ from clay-exchange sites; the displaced Na⁺ leaches with irrigation water through the soil profile and out via sub-surface drainage. The chemistry:
2 Na-clay + CaSO₄ → Ca-clay + Na₂SO₄ (leachable)
The dose depends on (a) how much Na is on the exchange complex (a function of initial ESP × CEC), (b) the target ESP you want to reach, and (c) the soil mass to be treated (bulk density × depth). The standard CSSRI Karnal formula in this calculator:
GR (t/ha) = ((ESPi − ESPf)/100) × CEC × 0.86 × BD × Depth × 0.1
The factor 0.86 is the equivalent weight of gypsum divided by 100 (1 cmol(+) requires 0.86 g pure gypsum to displace).
Worked example: Etawah, UP sodic flat
ESP 35, target ESP 15, CEC 18 cmol/kg, bulk density 1.4 g/cc, depth 15 cm. Calculator: (35 − 15)/100 × 18 = 3.6 meq/100g; gypsum pure = 3.6 × 0.86 × 1.4 × 15 × 0.1 = 6.5 t/ha. With 80%-pure mined gypsum, actual application = 6.5 ÷ 0.8 = 8.1 t/ha. Cost at ₹2,500/t = ₹20,250/ha for gypsum alone, plus ₹4,000-6,000/ha for the surface drainage channels CSSRI recommends to evacuate the leachate.
The reclamation timeline is 18-30 months: gypsum applied pre-monsoon, leached by monsoon rains + 2-3 supplementary irrigations, ESP drops to ~20 in season 1. A second half-dose in year 2 brings ESP to target. Crops follow this sequence: dhaincha (green manure, sodic-tolerant) in year 1 monsoon → KRL-1-4 / KRL-210 salt-tolerant wheat in year 1 rabi → CSR-30 salt-tolerant basmati in year 2 → normal cropping from year 3. ICAR-CSSRI Karnal's long-term Karnal Saline-Sodic Plot demonstrates economic sustainability: cumulative net gain after 10 years exceeds reclamation cost by ₹2.5-3 lakh/ha at 2024 prices.
Gypsum supply chain in India
India produces 4-5 million tonnes of gypsum annually, dominated by Rajasthan (Bikaner, Nagaur, Jaisalmer, Barmer — Hindustan Zinc / RSMM mines), Gujarat (Kutch — CSMCRI documented major sodic-coast reserves), Tamil Nadu (Nilgiris), and Uttaranchal. Two varieties: mineral gypsum (mined, 70-85% purity, ₹2-3/kg bulk) and by-product gypsum (phospho-gypsum from fertiliser plants, 90-95% purity, often radioactive due to uranium-238 series — restricted use). The Department of Land Resources, MoRD, runs a sodic-land reclamation programme under RKVY-RAFTAAR with partial state-level subsidies on gypsum (up to 75% in UP and Haryana under the Usar Reclamation Programme).
What this calculator does not capture
- Drainage requirement: gypsum without drainage is useless — Na⁺ has nowhere to go. Sub-surface tile drains at 0.9-1.2 m depth + 30 m spacing are CSSRI-recommended for permanent reclamation.
- Water quality: irrigating with Na-rich tubewell water (SAR > 6) re-sodifies the field. Use canal water or low-SAR sources during reclamation.
- Boron: B-toxicity often co-occurs with sodicity in arid Rajasthan; additional leaching needed.
- Organic amendment: 10 t/ha FYM accelerates structure recovery and microbial activity post-reclamation.
- Crop sequence: salt-tolerant CSR/KRL varieties critical in transition years.
Saline vs sodic vs saline-sodic
Confused terminology fixed: a saline soil has high soluble salts (EC > 4 dS/m) but low ESP — gypsum doesn't help here, only leaching with low-salt water does. A sodic soil has high ESP (> 15) but EC < 4 — gypsum is the correct intervention. A saline-sodic soil has both — gypsum first, then leaching. Use the Soil Test Interpreter to classify before deciding.
Reading the calculator output
The widget returns three numbers: meq/100g cation-exchange-replacement needed, equivalent pure gypsum in t/ha, and the actual application after purity adjustment. For severe sodicity (ESP > 40, CEC > 25), doses can exceed 15 t/ha — implement in two-year split to allow drainage. For mild sodicity (ESP 16-20), single 3-5 t/ha application often suffices.
Sources
ICAR-CSSRI Karnal Manual on Reclamation and Management of Salt-Affected Soils (2023); Abrol I.P. & Bhumbla D.R. (1979) Sodic Soils of the Indo Gangetic Plain; CSIR-CSMCRI Bhavnagar gypsum-reserve mapping 2022; Department of Land Resources, MoRD, Usar Reclamation Operational Guidelines 2024.